1. Field of the Invention
The present invention relates to computer memory, and more specifically to forming substantially void free crystalline phase change material in phase change memory cells.
2. Description of Background
There are two major groups in computer memory: non-volatile memory and volatile memory. Constant input of energy in order to retain information is not necessary in non-volatile memory but is required in the volatile memory. Examples of non-volatile memory devices are Read Only Memory (ROM), Flash Electrical Erasable Read Only Memory, Ferroelectric Random Access Memory, Magnetic Random Access Memory (MRAM), and Phase Change Memory (PCM); non-volatile memory devices being memory in which the state of the memory elements can be retained for days to decades without power consumption. Examples of volatile memory devices include Dynamic Random Access Memory (DRAM) and Static Random Access Memory (SRAM); where DRAM requires the memory element to be constantly refreshed while SRAM requires a constant supply of energy to maintain the state of the memory element.
The present invention is directed to phase change memory. In phase change memory, information is stored in materials that can be manipulated into different phases. Each of these phases exhibit different electrical properties which can be used for storing information. The amorphous and crystalline phases are typically two phases used for bit storage (1's and 0's) since they have detectable differences in electrical resistance. Specifically, the amorphous phase has a higher resistance than the crystalline phase.
Chalcogenides are a group of materials commonly utilized as phase change material. This group of materials contain a chalcogen (Periodic Table Group 16/VIA) and another element. Selenium (Se) and tellurium (Te) are the two most common elements in the group used to produce a chalcogenide semiconductor when creating a phase change memory cell. An example of this would be Ge2Sb2Te5 (GST), SbTe, and In2Se3.
Altering the phase change material's state typically requires heating the material to a melting point and then cooling the material to one of the possible states. A current passed through the phase change material creates ohmic heating and causes the phase change material to melt. A balance between the ohmic heating and heat flow produces a melting region (also referred to herein as a “switching region”) whose cross section minimum is defined by the diameter of the bottom electrode. In the latter, melting and gradually cooling down the phase change material allows time for the phase change material to form the crystalline state, while melting and abruptly cooling the phase change material quenches the phase change material into the amorphous state.
A problem in phase change memory is that voids may be dispersed in crystalline phase change material. Since phase change material shrinks in the crystalline phase and expands in the amorphous phase, voids can form throughout the phase change material as the phase change material crystallizes during cell construction. As a result, the properties of the phase change material, such as resistance and uniformity, become erratic.